Log-periodic antenna array having closely spaced linear elements



March 24, 1970 Filed Dec. 5, 1968 D. L. CARTER 3,503,074 LOG-PERIODICANTENNA ARRAY HAVING CLOSELY.SPACED LINEAR ELEMENTS 5 Sheets-Sheet lINVENTOR March 24, 1970 D. L. CARTER 3,503,074

LOG-PERIODIC ANTENNA ARRAY HAVING CLOSELY SPACED LINEAR ELEMENTS FiledDec. 5, 1968 3 Sheets-Sheet 2 I: 5 E I: E 5

. l l i i I 5| A 2 3 4 5 6 7 8 9 IO I5 20 FREQUENCY IN MEGACYCLES/SECONDINVENTOR DUNCAN L. CARTER March 24, 1970 D. CARTER 3,503,074

LOGPERIODIC ANTENNA ARRAY HAVING CLOSELY SPACED LINEAR ELEMENTS FiledDec. 5, 1968 5 Sheets-Sheet 5 STANDING WAVE i 9/3110 4 5.0 T 0 I60 RabINVENTOR DUNCAN L. CARTER United States Patent 3,503,074 LOG-PERIODICANTENNA ARRAY HAVING CLOSELY SPACED LINEAR ELEMENTS Duncan L. Carter,P.0. Box 653, Denham Springs, La. 70726 Continuation-impart ofapplication Ser. No. 563,892,

July 8, 1966. This application Dec. 5, 1968, Ser.

Int. Cl. H01q 11/10 US. Cl. 343-7925 10 Claims ABSTRACT OF THEDISCLOSURE A frequency independent antenna having a plurality of closelyspaced elements of unequal but predetermined related lengths, andcoupling means for connecting these elements in a manner so as toprovide phase reversal of current in alternate elements. The spacing maybe as close as physical limitations will permit so long as the elementsdo not contact each other.

This invention relates generally to antennas and more particularly toantenna structures which are substantially frequency independent over awide-band operating range and is a continuationin-part of vU.S. patentapplication Ser. No. 563,892, filed July 8, 1966, now abandoned.

One of the common problems relative to the use of antennas which areintended to operate over a wide-band range is the fact that a largenumber of antenna elements are required which each relate to aparticular frequency range. Normal antenna construction techniques,therefore, result in large, bulky and heavy antennas.

The present invention provides a compact operative antenna which issubstantially frequency independent over a very wide-band operatingrange.

Basically the invention comprises an antenna having a plurality ofclosely spaced antenna elements of unequal but predetermined relatedlengths, and coupling means for connecting these elements in a manner soas to provide phase reversal of current in alternate elements.

The invention will be more clearly understood from the followingdescription when taken together with the drawings wherein:

FIG. 1 shows a symmetrical dipole;

FIG. 2 shows an asymmetrical half-structure dipole;

FIG. 3 shows a schematic illustration of one type of coupling networkfor the antenna of FIG. 1;

FIG. 4 shows one type of coupling network which may be used with theantenna of FIG. 2;

FIG. 5 shows a monopole structure made in accordance with the presentinvention and encased in a dielectric material;

FIG. 6 discloses a coupling network which may be used with the antennaof FIG. 5;

FIG. 7 shows graphically the results obtained from one illustrativeantenna of the present invention;

FIG. 8 is a schematic illustration of a two element end fire arrayantenna;

FIG. 9 is a schematic illustration of the antenna from which the resultsshown in FIG. 7 were obtained; and

FIG. 10 depicts graphically the impedance curves obtained from actualtests of one of the antennas of the present invention.

Although not limited thereto, the present invention is described byillustrative examples which may be included in the linear log-periodicantenna structures. The basic inventive concept however may be appliedto various other types of antennas as will be apparent from thefollowing description.

3,503,074 Patented Mar. 24, 1970 For almost all current design oflog-periodic dipole antennas, the impedance of the individual elementsis not considered. Generally, the impedance problem is solvedempirically without any real understanding of the solution of theproblem, except for some cases involving shunt fed elements. Most dipolearrays are designed by selecting parameters of scale factor and spacingwhich give the desired directivity according to standard references(Carrel, R. L., Analysis and Design of the Log- Periodic Dipole Antenna,University of Illinois, Antenna Laboratory Technical Report No, 47, July15, 1960), and then adjusting the feedline impedance until satisfactoryimpedance characteristics are obtained; this is not always asatisfactory method. For the structures which have close element spacingand/or scale factors very close to one, the element impedances approachvalues which often do not yield satisfactory results when the structureis handled conventionally.

The variation of mutual coupling between adjacent elements is usuallyconsidered only as a function of the spacing between two parallel linearhalf-wave dipole antennas, (see pp. 262-272, Kraus, J. D., Antennas, Mc-Graw-Hill, New York, 1950). For the case of the thin center fedside-by-side /2 wavelength antenna, the selfresistance minus the mutualresistance approaches zero as the spacing between the elements goes tozero, a fact which has discouraged the construction of very closelyspaced, (less than M20), dipole arrays. However, that applies only todipole elements of the same length. Kraus has one short paragraphtitled, Parallel Antennas of Unequal Height, in which he cites areference, Cox, C, R., Mutual Impedance Between Vertical Antennas ofUnequal Heights, Proc. I.R.E., 35, 1367-1370, November 1947.

For closely spaced elements of unequal length, the difference betweenthe self resistance and the mutual resistance of the elements willalways be greater than zero, even if the spacing between the elementsapproaches zero. Consider the two element end fire array of very closespacing as shown in FIG. 8. The elements 61 and 63 are spaced by adistance S with overall length shown as L and L For the case of equalelement lengths, the structure would resemble two open circuitedtransmission line sections of length L/ 2; for which the feedpointresistance would approach zero as the spacing approaches zero.

For unequal element lengths as shown, it is apparent that if the twoelements are placed in the same alternating current electromagneticfield, it is impossible for currents of both equal magnitude and phaseto be induced in the elements of the antenna. In the region where oneelement is longer than \/2 and the other element is shorter than M2, theresultant currents at the feedpoint would tend to add, since the phasereversal caused by the transposition of the short feedline would becounteracted by the fact that one element would be cap'acitivelyreactive and the other element would be inductively reactive.

The term closely spaced insofar as the present invention is concerned,means that the individual conductors of the radiating element are placedtogether in a sufficiently small region of space that, as far as thedetermination of the essential radiation characteristics, the separateconductors occupy the same infinitesimal line in space. In other words,the only limit as to spacing is that the distance between individualconductors is limited only by the physical construction, that is, thespacing is only limited by the requirement that the conductors do nocontact each other.

Therefore, this type of frequency independent antenna requires only onephysical dimension, length, to define its physical characteristicsrather than description in two or three dimensions which all otherfrequency independent antennas require. In the prior art any use of theword closely means a spacing of adjacent elements somewhere in thegeneral region of A, to 5 of a wavelength, almost never more or less. Ifthese structures have spacings which are made appreciably less than lwavelengths, they cease to function as frequency independent structures.The parameter of spacing is an important parameter; either stated orimplied in all cases. In none of the prior teachings can the spacing bemade either very close or very far, that is outside the general rangegiven above and still allow the antenna to function. The prior art alsorequires that the conductors lie in an approximation of a plane and thatif the lengths of the elements are tapered they should be taperedsuccessively as they progress along the plane. The linear log periodicantenna of the present invention has no such requirement. The elementsmay be bunched together in a random grouping, though derivation of thenetwork is simplified if the conductors are arranged in some orderlymanner.

As an example, the prior art relating to the physical sizes of logperiodic antennas is as follows. The smallest example of the planar logperiodic array in terms of size vs. wavelength for the lowest frequencywhich is commercially available, relates to a structure which isdesigned to cover a frequency range of 3 to 30 mHz. It is a halfstructure and outlines an area of about 17-6 by 100 feet. The samefrequency range could be covered by a linear log-periodic dipole similarto the one described in the present invention having a length of 120feet and a thickness varying from A of an inch to 5 of an inch, areduction of size of about 2400 times. A linear log-periodic dipolecould be built with much smaller copper wire giving an element structureno larger in the center than a single strand of #20 common hookup wire.

In FIG. 1 there is illustrated a coupling network between two identicalelement structures so as to form a normal dipole. For purposes ofclarity the elements extending to one side of the coupling network arenumbered 13 through 29 with the opposing elements being a mirror imageof the elements so numbered.

Referring to FIG. 3, there is shown a coupling network composed of aplurality of coils, two of which have been labeled 22 and 24 andassociated capacitor-s 2-6 and 28. As will be obvious, this couplingnetwork provides an approximate phase reversal of current in theadjacent elements.

Generally, the spacing between each individual element will bedetermined by mechanical or by voltage breakdown requirements asdiscussed above and not by any pattern shaping requirements. The elementspacings and the current magnitude and phase differences between theindividual elements produce resultant far fields similar to a simplehalf-wave dipole at each frequency in the range covered by the antenna.Thus, the spacing between the individual elements of the frequencyindependent dipole as shown in FIG. 1 is small, and almost negligible ascompared to the phase shift introduced by the coupling network.

The scale factor, '7', representing the relativelength of the individualelements, is a function mainly of the band width of the individualelements and the desired band width of the entire antenna array. As anexample, a scale factor for a structure consisting of very short loadedelements might be 0.999 and for a multicone conical monopole or dipole,the scale factor might be 0.500. Likewise, a simple dipole might havescale factors varying from 0.7 to 0.9.

The asymmetrical half-structure dipole of FIG. 2 illustrates a furtherembodiment of the invention. In effect, the structure represents onehalf of the structure of FIG. 1 with adjacent individual elements ofFIG. 1 being reversed to extend on the opposite side of the couplingnetwork 30. For illustrative purposes, the elements are numbered 31through 47 in the order of their length extending outwardly from thecoupling network 30. One such coupling network which may be used isshown schematically in FIG. 4 with the inductive coils indicated at 32and 34 together with the capacitive elements 36 and 38 which again givethe desired phase reversal.

FIG. 5 illustrates a frequency independent monopole with specificstructural elements. Again the coupling network has the various elementssuch as 55 and 57 extended to the necessary matching number. In thisparticular embodiment, a flat cable with thin wire elements encased in adielectric material may be adapted for use with the antenna. The variousstepped structures 53 shown in FIG. 5 result from the cutting of thecable so as to conform to the desired relationship between theindividual antenna elements. The coupling network 50, which may be usedwith a structure such as shown in FIG. 5, is illustrated in FIG. 6 withthe various inductors and capacitors connected as shown and the antennaelements connected oppositely as indicated at 55 and 57.

It will now be obvious that the basic cell of the antenna structure ofthe present invention consists of two elements of unequal length closelyspaced and fed so that there is an approximate phase reversal of currentin the adjacent elements. A wideband structure can be constructed byrepeating this process somewhat indefinitely and connecting the elementswith an appropriate coupling network. As is current practice for otherlogperiodic structures, the highest frequency elements have been shownas connected closest to the feedpoint. The network, including the wireelements, must function as a transmission line between the activeelements of the structure and the feedpoint and must function as animpedance matching network in the active region of the antenna.

It should be noted that, in the drawings, the structures are shown asseries fed. However, the same technique applys to shunt fed structures.In practice, the choice between series and shunt fed structures ismainly a mechanical convenience since, electrically, the two types ofconnections are almost the same.

FIG. 7 shows the frequency independence of the present invention withthe graph being made from a symmetrical structure of thin wire dipoleelements placed together to form a cable of about /2 inch in diameternear the center of the antenna and tapering to about of an inch at theends of the antenna. Such a structure is shown diagrammatically in FIG.9 with the groups of elements 71 and 73 extending outwardly in oppositedirections from the coupling network 75. A scale factor, r, of the fifthroot of /2 or approximately .87 was used, with 18 elements varying inlength from 151 feet down to 14 feet. Some higher mode radiation occursproducing a slight sharpening of the dipole pattern above 10 me. Thishigh order radiation can be controlled to some extent by varying theantenna parameters and by placing traps in the individual elements.

FIG. 10 is a partial representation of a Smith chart relating to thedevelopment of an antenna as set forth in the above disclosure.

Curve A shows the impedance of a single element, such as schematicallyshown in FIG. 8, in the region where it is useful as a radiating elementin the linear log periodic dipole antenna.

Using FIG. 6 as an illustrative example, Curve B illustrates thetransformed impedance resulting from the addition of the shuntcapacitance between terminal 57 and ground. Curve C illustrates thefurther transformation resulting from the addition of the seriescapacitance in the network, and Curve D represents the resultantimpedance over a particular frequency range which occurs as a result ofthe use of a plurality of elements and associated networks. All of theabove curves include plotted points representative of the particularfrequency 5 in mHz. used in the tests. Such curves were used to plot theVSWR curve of FIG. 7.

The antenna of the present disclosure is the only type of log-periodicantenna which develops no increase in directivity or gain as compared toa simple dipole antenna. To my knowledge, every attempt to reduce any ofthe existing log-periodic arrays to a linear dipole array Of very closespaced elements has failed.

It is to be understood that the above description and drawings areillustrative only and that the basic approach disclosed herein may beapplied to short loaded antennas, to VHF dish feeds, to VLF systems, toelements of planar log-periodic arrays as shown, or, in short, to a verylarge portion of the antenna field. Accordingly, the invention is to belimited only by the scope of the following claims.

I claim:

1. A frequency independent dipole antenna comprising at least threedipole elements of decreasing length, the

relative length of individual elements being determined by asubstantially constant scale factor 1- Which is a function of thebandwidth of the individual elements and the predetermined bandwidth ofthe entire antenna,

the spacing between said individual dipole elements being substantiallyless than & :wavelength and being determined by mechanical or voltagebreakdown requirements,

coupling network means comprising lumped inductive and capacitivereactances feeding said elements in a manner such that there is a phasereversal of current in alternate elements such as to produce resultantfar fields substantially identical to a simple half-wave dipole at eachfrequency in the range covered by the antenna, and

means for connecting a transmission line to the smallest of said dipoleelements.

2. The antenna of claim 1 wherein said elements are mounted in a planararray.

3. The antenna of claim 2 wherein said elements are encased in adielectrical material.

4. The antenna of claim 1 wherein said elements form a symmetricaldipole array.

5. The antenna of claim 1 wherein said elements form an asymmetricalhalf-structure dipole array.

6. The antenna of claim 1 wherein said elements are mounted so as toform a cable.

7. The antenna of claim 6 wherein said elements are encased in adielectric material.

8. A frequency independent monopole antenna comprising at least threelinear elements of decreasing length, the

relative length of individual elements being determined by asubstantially constant scale factor 1- which is a function of thebandwidth of the individual elements and the predetermined band-width ofthe entire antenna, the spacing between said individual elements beingsubstantially less than wave ength and being determined by mechanical orvoltage breakdown requirements, coupling network means comprising lumpedinductive and capacitive reactances feeding said elements in a mannersuch that there is a phase reversal of current in alternate elementssuch as to produce resultant far fields substantially identical to asimple half-wave dipole at each frequency in the range covered by theantenna, and means for connecting a transmission line to the smallest ofsaid elements. 9. The antenna of claim 8 wherein said elements aremounted in a planar array.

10. The antenna of claim 9 wherein said elements are encased in adielectric material.

References Cited UNITED STATES PATENTS 2,192,532 3/ 1940 Katzin 34381l2,433,804 12/ 1947 Wolff 343811 3,389,396 6/1968 Minerva et al. 343-8113,392,399 7/1968 Winegard 343--815 3,396,398 8/1968 Dunlavy 343-844FOREIGN PATENTS 34,357 1/ 1965 Germany.

ELI LIEBERMAN, Primary Examiner U.S. Cl. X.R. 343814, 873

